This invention relates to a mainlobe jamming cancellation system which can be used to allow a radar to detect targets even when relative motion exists between the jammer and the radar platform. In particular, this invention relates to a mainlobe canceller that utilizes a main antenna and one or more widely spaced auxiliary antennas in conjunction with one or more phase locked loops (PLL), to automatically align the jamming signals received by the several antennas. The alignment is necessary to realize a high degree of jammer cancellation.
A radar system can be prevented from detecting a target by the use of jammers which raise the background noise level received by the radar. The noise level is increased sufficiently so that the radar cannot see or distinguish the reflection or echoes of the pulse reflected from the target. The radar system uses a main antenna to transmit and receive the radar pulse. This antenna has a mainlobe and sidelobes. The mainlobe has a narrow angular extent while the sidelobes cover the remainder of the hemisphere.
Interference jamming can come from sidelobe or from mainlobe directions. The jammer may be located on board an object accompanying a weapon carrying vehicle. For example, there may be two aircraft, one of which carries a weapon while the other carries a device used to generate the jamming signals. In the alternative, the jammer or the interfering signal could be mounted on a platform on the ground or it could be on a satellite platform in space. If the jammer is to jam the radar signal in the mainlobe its signal must be located within the appropriate narrow angular region. In fact, it must be sufficiently close to the radar target so that, when the radar signal illuminates the target, the jammer will also be within the mainlobe region of illumination. The angular extent of the mainlobe (half power points on the mainbeam) of the radar's antenna can be expressed as .lambda./D, where D is the diameter of the antenna and .lambda. is the wavelength at the center frequency within the radar's operating bandwidth. Thus, the separation between the jammer and the target must be less than R.lambda./D, where R is the range to the jammer, if the jammer and target are to simultaneously fall within the mainlobe.